Degradation Mode

A specific process or phenomenon that causes degradation of material properties, leading to eventual failure of equipment. Damage mechanisms include corrosion (uniform, localized, galvanic), erosion, fatigue, creep, hydrogen damage, stress corrosion cracking, embrittlement, and other metallurgical or environmental attack modes. API 571 catalogues over 60 damage mechanisms relevant to refining and petrochemical equipment.

The rate of material loss due to corrosion or erosion, typically expressed in millimeters per year (mm/yr) or mils per year (mpy). Short-term corrosion rate is calculated between the two most recent thickness measurements; long-term corrosion rate is calculated from the original (or earliest) thickness to the most recent measurement. The more conservative (higher) rate is typically used for remaining life calculations.

A group of analysis techniques used to evaluate the condition of materials, components, and assemblies without causing damage or altering their serviceability. NDE methods detect surface and subsurface flaws, measure wall thickness, characterize material properties, and assess the integrity of welds and joints. Common NDE methods include ultrasonic testing (UT), radiographic testing (RT), magnetic particle inspection (MPI/MT), liquid penetrant testing (PT), visual testing (VT), eddy current testing (ECT), and acoustic emission testing (AE).

A qualitative rating of the probability that an inspection activity will correctly identify and characterize the damage present in the inspected equipment. Inspection effectiveness is categorized from A (Highly Effective — high probability of detecting the damage) through E (Ineffective — virtually no chance of detection). The rating depends on the inspection technique used, the skill of the inspector, the coverage area, and the damage mechanism being sought.